1. Field of the Invention
The present invention relates generally to an injection molding apparatus and and method of injection molding and, in particular, to an apparatus and method that prevents or limits leakage of molten material between a hot runner nozzle and a manifold.
2. Background of the Invention
A common problem associated with hot runner injection molding systems is the leaking of molten plastic that can occur between the manifold and the nozzle assembly. This is undesirable because the leaked material often remains in the injection molding system and crystallizes or degrades and as a result may contaminate subsequent injection molding operations. In addition, the leaking of material may cause servicing of the injection molding system to be more difficult.
Another problem with leaking is that some materials are corrosive or otherwise aggressive to materials within the nozzle assembly. The continued presence of such materials can result in decreased lifetimes of the components.
The components of the injection molding system are subjected to thermal expansion and contraction during each molding cycle as well as during start up and shut down of the system. The materials and clearances of parts within the injection molding system must be selected to reduce leaking within the operating temperature range since different components often exhibit different thermal expansion due to material properties and specific dimensions. However, since the parts are generally designed for a specific operating temperature range, operation outside this operating temperature window often results in unacceptable leaking.
In order to accommodate various plastic materials and/or injection processes, it is desirable for the available operating temperature range of the injection molding system to be as large as possible. A narrow operating temperature range limits the use of the injection molding system to specific materials and processes.
Leakage of molten material at the interface between a manifold and nozzles of an injection molding apparatus may also occur because of bending of the manifold caused by the injection pressure. A small gap between a lower surface of the manifold and a head portion of a nozzle may form when a manifold bends providing a path for leakage of molten material. The size of gaps between the manifold and nozzles in an injection system may be different for each nozzle and as a result leakage from those gaps is difficult to control using the current state of the art nozzle designs.
Therefore, there is a need to reduce the occurrence of leaking between the manifold and the nozzle assembly for a variety of molding conditions and for large operating temperature ranges.